In an airplane, electricity is supplied by an alternator that is driven by the engine of the airplane. The alternator delivers an alternating voltage that is converted into a DC voltage for use by on-board electrical equipment in the airplane.
In its simplest version, conversion is performed by means of a transformer having a primary winding connected to the alternator and a secondary winding connected to a rectifier bridge associated with a filter capacitor. The output voltage from the rectifier bridge is a fullwave rectified sinewave, and current consumption is subjected to a high level of distortion that gives rise to a drop in the efficiency of the transformer and of the alternator, to heating of the conductors, and to high-frequency electromagnetic radiation that gives rise to interference.
One way of remedying that drawback is to provide filtering in series with the primary circuit of the transformer. Nevertheless, that option is not appropriate when the frequency of the alternating voltage is variable, as happens with an alternator driven by a jet turbine for which the frequency varies over the range 360 hertz (Hz) to 800 Hz, approximately.
It is also known to have recourse to a power factor corrector (PFC) circuit for reducing distortion by forcing current consumption to follow a waveform identical to that of the input voltage, i.e. a fullwave rectified sinewave. There are various different structures of PFC.
In a “boost” type structure, the circuit does not have electrical isolation which makes it necessary to associate the circuit with a DC/DC converter that provides this function. That assembly provides overall efficiency that is relatively low, and also high levels of bulk and weight.
In the so-called “flyback” type structure, the circuit includes an isolating transformer with primary and secondary windings wound in opposite directions. The operation of PFC circuits of this type leads periodically to a large quantity of energy being stored in the magnetic core of the transformer. It is therefore necessary to use large transforms for high powers, thereby increasing the weight and the bulk of the circuit.
In the structure of the so-called “forward” type, the circuit also includes an isolating transformer, but the primary and secondary windings are wound in the same direction. In this type of circuit, it is not possible to make use of the current over all of the resulting sinewave and thus to consume current with the same waveform as the input voltage.